A thin film containing a metal oxide such as tin oxide has the function of reflecting infrared rays. Since a glass sheet provided with this thin film reduces the total solar energy transmittance and does not allow the heat within rooms to escape to the outdoors, it is widely available in the market as a low-emissivity glass. This thin film also can exhibit the function of shielding electro-magnetic waves. A known method for manufacturing a glass sheet of this kind is such that a thin film of a metallic compound is formed on a high temperature glass surface utilizing thermal decomposition methods, such as a chemical vapor deposition method (CVD method) and a spraying method in which a solution material or a solid material is sprayed.
For example, JP 11(1999)-509895 A describes a method of forming a thin film of a tin oxide by supplying a gaseous reaction mixture containing an organic tin compound, hydrogen fluoride, oxygen, and water onto a high temperature glass surface. JP 6(1994)-47482 B describes a method of forming a thin film comprising a tin oxide by supplying a vapor of an organic tin compound on a glass ribbon surface in a float bath in a float manufacturing process. The use of the organic tin compounds such as described in these patent publications as a raw material for a thin film has an advantage that the thickness of the thin film easily is made uniform. Nevertheless, because organic tin compounds have high environmental loads as with tributyltin compounds, the use of alternative raw materials that replace organic tin compounds has been desired in recent years.
Meanwhile, tin chloride conventionally has been used widely as a raw material for a tin oxide thin film in thermal decomposition methods. For example, JP 2(1990)-175631 A describes a method of forming a coating film in which, with a CVD method, a first flow of tin tetrachloride and a second flow of water vapor are supplied onto a glass with a turbulent flow. Also, JP 9(1997)-40442 A describes a method of depositing a tin oxide thin film uniformly by a CVD method, in which tin tetrachloride and water are pre-mixed and supplied onto a glass substrate with a laminar flow.
Such methods of forming a thin film containing a metal oxide that utilize thermal decomposition methods are inferior to physical vapor deposition methods, such as a sputtering method, in that it is difficult to obtain a uniform film thickness; nevertheless, they are capable of forming a thin film over a wide area within a short time at a relatively uniform thickness and therefore are suitable for mass production of industrial products. With the thermal decomposition methods, generally, the higher the temperature of the reaction system is, the faster the film deposition rate, although the situations vary somewhat depending on the compositional components of the raw materials for the thin film. Accordingly, it seems that higher temperatures are preferable for the formation of a thin film in industrial production processes.
With the background of recent energy issues and environmental issues, solar cells have attracted attention. There are various types of solar cells, and among them, thin film solar cells have been considered as the mainstream henceforth in terms of resource savings. A general configuration of thin film solar cell is as follows. The structure is that a transparent conductive film composed of tin oxide (SnO2) or the like, a photoelectric conversion layer composed of a non-crystalline semiconductor such as amorphous silicon or amorphous silicon germanium, and a conductive film are stacked successively on a transparent substrate such as a glass sheet.
Solar cells constantly have been required to improve their photoelectric conversion efficiency, and various technologies have been developed and put into practical use for that purpose. A typical example is a technology for producing a so-called light trapping effect, in which the surface of a transparent conductive film is provided with surface roughness where incident light is scattered to lengthen the optical path length in the photoelectric conversion layer. Such surface roughness in the transparent conductive film is originated from crystal growth of tin oxide. In order to grow large crystal grains of tin oxide, it is effective to carry out a film deposition at high temperatures or to increase the thickness of the thin film. For example, JP 2862174 B describes an electrically-conductive film solar cell substrate that is formed by atmospheric-pressure chemical vapor deposition using SnCl4, H2O, CH3OH, and HF as raw materials, with numerous protrusions on the surface.
In addition, in order to enhance the photoelectric conversion efficiency of a solar cell, it is essential to increase the amount of light incident on the photoelectric conversion layer, and there have been developed a technology for reducing the reflectance for incident light and a technology for reducing the absorptance in the transparent conductive film. For example, JP 2001-35262 A proposes a thin film made of a tin oxide in which the absorption coefficient is suppressed to be low within such a wavelength range that a solar cell can utilize effectively.
In the above-noted thermal decomposition methods, however, the formation of the thin film virtually denotes crystal growth in the case where the metal oxide is crystalline; therefore, the conditions of the crystal growth in the thin film change considerably depending on the conditions of the surface of the substrate or the like on which the thin film is formed. Specifically, if what serves as starting points for crystal growth exists in large numbers on the surface of a substrate on which the thin film is formed, crystal growth starts from numerous points, and consequently, the crystal growth, that is, the thickness of the thin film, becomes relatively uniform. On the other hand, if what serves as starting points for the crystal growth exists in fewer numbers on that surface, each one of the crystals grows large before the formation of crystal nuclei in the case of a raw material with faster reactivity being used, and a variation in film thickness therefore becomes large. Moreover, crystals become more difficult to grow, slowing down the film deposition rate. In turn, if the formation temperature of the thin film is increased to complement the slowing down of the film deposition rate, the variation in film thickness becomes even larger.
For example, an example in JP 2(1990)-175631 A describes that a tin oxide thin film is formed at a glass temperature of 580° C. using tin tetrachloride. When a tin oxide thin film is formed under this condition, no problem arises in terms of the performance of the thin film except that the film deposition rate is slow; however, a further experiment performed by the present inventors proved that when the thin film was formed with the glass temperature elevated to 615° C., the glass surface was observed to have a white turbidity. When the portion with the white turbid condition was observed with an electron microscope, giant crystal grains that were as large as 2 μm in diameter was observed, together with the adjacent portions in which crystal grains are absent. This seems to be because, before crystal nuclei were formed uniformly on the glass surface, initially-formed nuclei had grown abruptly, forming a large unevenness on the glass surface, and thus, when macroscopically seen, the haze ratio (haze factor) considerably increased.
Further, Example 25 in JP 9(1997)-40442 A describes that, with a manufacturing process for a glass sheet using a float process, an undercoating film made of silicon oxide was deposited on a glass ribbon using a CVD method, and subsequently a thin film made of tin oxide was formed at a film deposition rate of 6234 nm/min. However, since an undercoating film made of silicon oxide has a very smooth surface, it is obvious that when a thin film made of tin oxide is deposited at a film deposition rate of as fast as 6234 nm/min. without performing any treatment thereto, giant crystal grains should form, causing a white turbidity. In this regard, JP 9(1997)-40442 A contains no description concerning the surface condition of the glass substrate of Example 25.
On the other hand, in JP 2862174 B, in which its object is to further grow crystal grains of tin oxide using tin chloride as a raw material and to increase the film deposition rate to improve productivity, a transparent conductive film with a thickness of 350 nm or greater is deposited by a thermal decomposition method on a surface of a glass substrate the temperature of which is higher than 615° C. When a film deposition is carried out at such a high temperature, crystal grains are not uniformly formed in the surface, resulting in a white, turbid transparent conductive film with a very high haze ratio. There has been a problem that if a photoelectric conversion device is constructed by an amorphous silicon layer formed on this transparent conductive film, the amorphous silicon film, which serves as the photoelectric conversion layer, is not uniformly formed, and the efficiency of the solar cell is reduced.
Furthermore, there has been a problem when dimethyltin dichloride or monobutyltin trichloride is used as tin materials other than those described above, in that although the white turbidity does not occur, the absorptance becomes large in the wavelength range of 400 to 700 nm, in which the light quantity of solar light spectrum reaching the Earth's ground is large, particularly in the short wavelength range thereof, and as a result, the incident light volume on the photoelectric conversion layer in the solar cell is small. For example, JP 2001-35262 A describes absorption coefficients of the tin oxide films using dimethyltin dichloride or monobutyltin trichloride as a raw material; the absorption coefficient thereof is lowest at a wavelength of about 600 to 700 nm, and the absorption coefficient at 400 nm is 1.8 times or greater than that of the foregoing range.